U.S. Pat. No. 6,796,185 introductorily describes a differential pressure sensor based on older technology, wherein from a high-pressure side and a low-pressure side (actually two high pressures, with one pressure being higher than the other), two different pressures are transferred to two trapped oil volumes in channels within the sensor. A desire exists to protect an internally located differential pressure sensor (a) from being exposed to possible aggressive chemical substances, and (b) from being exposed to an extreme differential pressure should one of the two external pressures suddenly drop to zero, for example. Accordingly, in consideration of a), internal oil volumes are used, so that the internal pressure sensor is located solely in an oil environment. As to b), a favorable design using diaphragms with a deflection restriction ensures that an overpressure event cannot be transferred to the internal pressure sensor. The older technology shown in FIG. 2 of the US Patent provides definitions of the diaphragms included: Two outer diaphragms, being exposed to the external process environments which differential pressure is to be measured, is referred to as “process diaphragms.” These diaphragms separate the internal oil environments within the sensor from the external process environments and transfer the two pressures into the sensor. Inside the sensor, a diaphragm is located that separates the two oil volumes/pressures from each other and that is able to flex in both directions inside a chamber. This diaphragm is referred to herein as a “separating disc.”
From JP 4-299230 a differential pressure sensor is known comprising two diaphragms on a high-pressure side and one diaphragm on a low-pressure side. A pressure sensor is connected to one of the diaphragms on the high-pressure side, and from the pressure sensor a channel runs that branches to the other high-pressure side diaphragm and to the diaphragm on the low-pressure side. In reality, this pressure sensor works according to a rather simple differential measurement principle. All three diaphragms are process diaphragms, and it has to be assumed that only a single separating disc is used, such as in the prior art referred to above.
U.S. Pat. No. 6,796,185 then describes an improvement of the above technology, still comprising two outer process diaphragms exposed to the external environments, but wherein additionally two inner separating discs are used. Each of these discs is only able to deflect in one direction, as both discs bear against a restricting surface. That is, in this case the pressure transferring mechanism is a diaphragm assembly that (from each process pressure side) is comprised of an outer, planar process diaphragm transferring pressure from a process to the oil volume and a pre-tensioned, corrugated separating disc bearing against a concave abutment face within the sensor. The corrugated separating disc is pre-tensioned inwards towards the abutment face. Both the high-pressure side and the low-pressure side have a transferring mechanism consisting of a process diaphragm and a corrugated separating disc. The diaphragm assembly of the high-pressure side is positioned facing the diaphragm assembly of the low-pressure side in a lower part of the sensor. The space between the separating disc and the process diaphragm on the high-pressure side is filled with oil, and is connected with the differential pressure sensor and backside of the corrugated separating disc on the low-pressure side through a channel. There is also a similar channel connection from the space between the separating disc and the process diaphragm on the low-pressure side (the space being filled with oil) to the differential pressure sensor and the backside of the corrugated separating disc on the high-pressure side. The problem with this differential pressure sensor is that it contains large internal oil volumes. At a change in the temperature or pressure, the volume of this oil will change, either due to thermal expansion or compression or a combination thereof. This volume change must be compensated for by the process diaphragms, so that these introduce a pressure drop that will deteriorate the measurements. With a smaller oil volume, this measurement inaccuracy will be reduced. This is particularly important when the line pressure is many times greater than the differential pressure.
The same volume change will cause the cutoff pressure to change. A smaller volume results in a smaller volume change, providing a more precisely defined cutoff pressure.
The publication WO 92/17757 also shows a differential pressure sensor comprising two separating discs, in a similar manner as in the above US Patent, and exhibiting similar problems with respect to internal oil volumes.